This invention relates to a charged particle beam device for the examination of specimens. In particular, this invention relates to the examination of specimens which have the tendency of getting charged while being radiated with a charged particle beam.
Negatively or positively charged particles coming from a particle source can be accelerated by a potential of V volts. The direction of travel of such a moving particle is altered by applying either a magnetic or an electric field; for example, a charged particle moving in a magnetic field experiences a force tending to change its direction of motion except when it is travelling parallel to the magnetic lines of force. Suitably shaped magnetic and electric fields can be used to cause charged particles diverging from a source to converge into a beam, guide the beam along a predetermined path and allow it to impinge on the surface of a substrate or a specimen.
The charged particles interact with the atoms of the specimen and cause a number of different effects in the specimen or on its surface. Without limiting the scope of the present invention, the following explanations will primarily concentrate on the use of electrons as charged particles. The impinging electrons, generally called primary electrons (PE), are deflected by collisions with specimen atoms. These collisions may be elastic when the electron is deflected (even up to 180 degrees), but no energy interchange occurs. They may also be inelastic when the primary electron interacts with the atom and supplies energy for a further process to occur. Such a process could result in the emission of an electron, called secondary electron (SE) and/or electromagnetic radiation. Thereby, the primary electron generally experiences only a small deviation of track. After the collision or collisions, the primary electron may re-emerge as backscattered electron (BSE) or as transmitted electron, or may lose all its energy and come to rest in the specimen. There, the primary electron contributes to the specimen""s heating or to the specimen""s absorbed charge.
The above indicated physical effects can provide much analytical information about the specimen. In the following, the creation of secondary electrons and their informational content of the specimen will be considered in greater detail. An inelastic collision of an incident primary electron having a kinetic energy of e.g. 1 keV, can result in electrons being detached from the specimen atoms. This leaves behind an ionized atom with a positive charge. The dislocated electrons have a low kinetic energy, typically less than 50 eV, and are easily captured by nearby atoms. Some electrons which are created closer to the surface can be emitted from the specimen and can be collected with specific detectors. Consequently, only a small portion of the secondary electrons formed are available for collection. Since the emitted SE originate from a small region very close to the surface of the specimen, they carry corresponding surface information.
In particular, a surface of the specimen which is tilted relative to the incident beam reacts differently than a surface perpendicular to the incident beam. Compared to a flat surface, the electrons having entered a tilted surface of the specimen propagate a longer distance close to the specimen""s surface. This results in a greater proportion of secondary electrons that are to escape, and so the electron emission from the surface will increase. The intensity of the secondary electron emission is therefore an indicator of the surface slope and topography. Therefore, the intensity signals collected by secondary electron detectors can be used for high-resolution surface imaging. Instruments visualizing these surface effects have become increasingly important for the development of e.g. microelectronic components. They are used to identify deviations from predetermined patterns or to evaluate topographical parameters such as height, width or angle of inclination of the structure under examination. An example of a widely used system is the scanning electron microscope (SEM). A critical dimension SEM (CD-SEM) is routinely used to measure dimensions of elements on a semiconductor wafer to a nanometer resolution.
It should be appreciated that in order to obtain an image using the SEM, the number of electrons in the primary beam must be different from the number of electrons emitted from the specimen (i.e., the yield must be different from 1). This is especially true for insulators and semiconductors where charge is easily accumulated. The charge can result in a strong electrical field prevailing at the surface of the specimen and substantially altering the image of its surface by, for example, altering the path of PE""s and SE""s. In a semiconductor device, for example, electric insulators such as SiO2 are often deposited on conductors such as Al or semiconductors such as silicon. When a PE beam is directed onto the device, the surface of the insulator is charged. The resulting electric field alters the direction of PE""s and SE""s and results in inaccurate measurement of the features. This problem is even more severe when several lines to be measured are closely positioned, so that many interactions with charge fields occur to cause deviation from the actual measurement. Additionally, such a field at the surface can prevent SE created at the bottom of a contact hole and vias from reaching the detector.
In the past, a variety of methods have been tried to solve these problems. The approaches included adaptation of the acceleration voltage and the current of the electron beam. Others have altered the scanning speed of the primary electron beam or modulated the primary electron beam before impinging on the specimen. However, these methods have not been satisfactory. In some cases the intensity of the emitted secondary electrons is too low, in other cases the results obtained by comparative measurements are unreliable.
In an alternative approach, Environmental Scanning Electron Microscopes (ESEM) have been used. Originally these instruments were developed for the examination of specimens which are sensitive to dehydration caused by the vacuum in the specimen chamber. The use of a low pressure environment in the chamber prevented the dehydration. As a secondary effect, the presence of ions in the irradiated gas impeded charging of the specimen. These ESEM systems, however, cause widening of the beam of charged particles due to the scattering of the primary electrons due to the absence of vacuum. Also, the high gas concentration in the electrical fields between the detectors and the specimen can result in arcing. Therefore, the conventional ESEM systems did not lead to satisfactory results either and cannot be used for semiconductor applications since such applications require high vacuum environment.
Various proposed methods to avoid charging in SEM examination of semiconductors are presented in U.S. Pat. No. 5,869,833. While the focus of that patent is various scanning schematics to prevent charge or cause discharge, there is also a mention of introducing gas into the vacuum chamber. However, the discussion relating to the introduction of gas is basically limited to a single paragraph and lacks many details needed to suggest a working system.
Accordingly, the present inventors undertook an in-depth study of the subject of semiconductor charging and, in particular, the use of gas discharge in a vacuum chamber. Thus, the present inventors uncovered many of the difficulties needed to be overcome in order to make such a system workable in practice. The various problems and solutions worked out by the present inventors are detailed herein.
The present invention intends to provide an improved apparatus and method for examining a specimen with a charged particle beam. Specifically, the various embodiments of the present invention utilize injection of gas into the vacuum chamber to assist in discharge of the specimen.
According to a further aspect of the present invention, there is provided an apparatus for examining a specimen with a beam of charged particles. The apparatus comprises a particle source for providing a beam of charged particles which propagate along an optical axis and an optical device for directing said beam of charged particles onto said specimen to be examined. Further, the apparatus comprises a gas supply for providing an inert gas to the area of incidence of said beam of charged particles onto said specimen, and a vacuum chamber for loading said specimen. According to one aspect of the present invention, the inert gas is provided in the form of a layer. This layer preferably covers the area of the specimen where the beam of charged particles is incident. In the context of this application xe2x80x9clayerxe2x80x9d is not understood as a geometrical object with clearly defined limits. Rather, the inert gas molecules forming the layer preferably have a higher concentration in the area of the specimen where the beam is incident. The concentration decreases along the beam with increasing distance to the specimen.
In a further preferred aspect of the present invention, a nozzle directs the stream of inert gas to the area of incidence. This could preferably be done by either varying the gas flow or by varying the pressure, or by varying both. Instead of a single nozzle two or more can be used to provide the inert gas. In such a case the nozzles are preferably arranged in a symmetrical pattern.
According to a still further aspect of the present invention, there is provided an electrode in the vicinity of the area of incidence of the charged particle beam. The electrode allows a controlled charging of the specimen. The inert gas will cause an exchange of charge between the specimen and the electrode. Advantageously, the electrode establishes a desired voltage level in a limited region of the specimen. Furthermore, it is preferred to provide means for positioning the electrode e.g. with an x-y-z manipulator (e.g., FIG. 1). Depending on the material or the surface structure of the specimen, the electrode can be placed at a specific location over the specimen with a specific distance from it.
In another embodiment according to the invention, the nozzle and the electrode are provided as an integral part. This can be achieved either by a common carrier or by attaching the electrode to the nozzle. Preferably the electrode is arranged in front of the nozzle. Both parts, the nozzle and the electrode, advantageously comprise a coupling device. This allows exchange with other nozzles or electrodes having different geometrical dimensions and therefore, different physical properties.
It is preferred to use an electrode creating an electrostatic field with a rotational symmetry. This symmetry limits the influence on the trajectory of the incident charged particle beam and the secondary electrons. In one such embodiment, the electrode is a circular ring electrode with the charged particle beam being guided through its center.
Advantageously, the electrode comprises a conductive net. At least one of the net meshes is used to let the particle beam pass. Although such an arrangement will slightly distort the rotational symmetry of the resulting electrostatic field, it improves the efficiency of the electrode in controlling the charging.
According to another aspect there are provided vacuum means capable of providing a pressure gradient in the specimen chamber. Preferably the pressure gradient is established along the incident particle beam with a higher pressure at the point of incidence and a lower pressure at the aperture for permitting the particle beam to enter the specimen chamber. The pressure gradient reduces the number of collisions between the particle beam and any atoms or molecules other than those in the specimen. Preferably the pressure gradient is established in such a way that the particles of the incident beam collide on the average less than once with an atom or molecule in the chamber before hitting the specimen.
Preferred inert gases used in the context of this application are N2, CO2, or SF6 or noble gases such as Argon. The gases used, however, are not limited to those mentioned above. Any other gas which is inert to reacting with the surface of the specimen can be used.
According to another aspect of the invention, inert gas is provided to the area of incidence in a discontinuous or pulsating manner. This allows reduction of the amount of inert gas in the specimen chamber. Inert gas is primarily guided to the area of incidence when it is needed for decharging.
According to a further aspect of the invention, the gas conduit is integrated into the objective lens or into any other mechanical parts close to the specimen. Such an arrangement allows for the use of existing set up of a microscope column and maintains the rotational symmetry around the optical axis.
According to a still further aspect of the present invention, there is provided a charged particle beam apparatus comprising a particle source and an optical device for directing a beam of charged particles onto said specimen to be examined. The apparatus further comprises a gas supply for providing a gas to the area of incidence of said beam of charged particles and a 2-way valve arranged in the gas supply whereby a first port of the 2-way valve is connected with a gas reservoir and a second port of the gas supply is connected with a vacuum reservoir. A control unit switches the 2-way valve.
The invention is also directed to methods by which the described apparatus operates. It includes method steps for carrying out every function of the apparatus. Furthermore, the invention is also directed to apparatus for carrying out the disclosed methods and including apparatus parts for performing each described method step. These method steps may be performed by way of hardware components, a computer programmed by appropriate software, by any combination of the two or in any other manner.